Fixed-gain intensifier tubes have been used for many years in a variety of night vision systems, including binoculars, binoculars, monoculars, and weapon sights. Variable-gain tubes have received more limited application due to the size, weight, cost, and complexity penalties associated with implementing the variable gain features and controlling gain limits. Variable gain tubes have found their greatest use in driver's viewers, weapon sights, cameras, and commercial systems such as the UT Night Mariner binocular, where minimized size and weight are not critical.
Variable gain can be implemented by adjusting the light level into or out of the tube via an arrangement of filters or irises, as is done in intensified camera systems, for example. Variable gain can also be implemented electro-optically by filters whose transmissivity is adjustable by means of an applied voltage or current. Variable gain can also be implemented electrically by adjusting the magnitude or duty factor of one or more of the power supply voltages to the tube module.
Generally, the intensifier tube's light gain increases with increases in magnitude in any of its supply voltages. In camera systems, the cathode or microchannel plate (MCP) voltage is often gated at a variable duty factor to achieve variation in the time-averaged light output. The MCP voltage is often varied to achieve variable gain, as this voltage can be varied with minimal adverse impact to other factors such as tube resolution and life. Most so-called fixed-gain intensifiers include automatic brightness control (ABC), which reduces the MCP voltage under control of the power supply when the screen current reaches a preset threshold, in order to prevent excessive output brightness in conditions of brighter input light. User-adjustable gain is typically achieved by varying the MCP voltage via push-button or rotary control inputs from the user.
In systems such as the Night Mariner for example, push-button inputs cause a stored number in system memory to increase or decrease. The stored number is used with a digital-to-analog converter which produces an output control voltage proportional to the number. This voltage is used in the intensifier power supply as a reference voltage, to which a divided version of the MCP voltage is compared in an error amplifier. The output of the error amplifier controls the circuit which produces the MCP voltage, such that the MCP voltage is a fixed multiple of the adjustable control voltage. In other implementations of the power supply, the control voltage sets a limit on the level of an AC voltage which is subsequently stepped-up and rectified to produce the variable MCP voltage.
In some systems, such as the MX-1 1620 and MX-1 1619 25-mm intensifier tubes, the gain is varied by means of an external gain adjustment circuit (EGAC) which is a part of the viewer system or weapon sight system in which the tube is installed. The EGAC includes a user-adjustable potentiometer to enable setting the gain to any value between established limits. Inside the tube housing are two additional potentiometers which permit the upper and lower gain limits to be factory-set to specified values.
These potentiometers are typically part of the feedback or control network for the power supply which regulates the MCP voltage. Adjusting a potentiometer changes the level of feedback which in turn causes the magnitude of the MCP voltage to change.
The smallest intensifier tube in regular production is the MX-10160 tube, and commercial versions of the same, for use in AN/AVS-6 and similar night vision systems. The MX-10160 contains an integral power supply, potted with the intensifier tube module, in a cylindrical housing. However, this tube does not offer variable gain. The power supply in the MX-10160 does however contain a screwdriver-adjustable potentiometer used to factory-set the tube gain to a specified value. This potentiometer is part of the feedback network that controls the MCP voltage in the power supply.
Customers increasingly desire the variable gain feature in a small, lightweight package, such as a monocular. In systems which have interchangeable tubes, customers prefer that any tube be installable in any system and yet retain the same maximum and minimum gain limits, with no required adjustments. The ability to replace or interchange tubes without requiring gain limit adjustments means that maintainers can do their work faster and with less support equipment, reducing maintenance costs. While systems such as the MX-1 1620 and MX-1 1619 25-mm intensifier tubes offer variable gain, set limits, and interchangeability, these tubes are heavy and bulky, not suitable for hand-held or head-mounted use. Further, the Night Mariner does not offer interchangeable tubes, uses a relatively complex gain adjustment circuit, and is also too heavy and bulky for head-mounted use. In addition, it does not have any ability to set a limit on minimum gain. Likewise, intensified camera systems with variable gain are generally too heavy and bulky for head-mounted use, and do not offer interchangeable tubes.